1. Field of the Invention
The present invention relates to a device for measuring temperatures by quantifying infrared emissions, and more particularly, to a device which measures patient body temperature by quantifying the infrared emissions from the tympanic membrane.
2. Description of the Prior Art
The tympanic membrane has long been known to be an excellent spot to check body temperature because it shares the blood supply that reaches the hypothalamus, the center of core body temperature regulation. Also, the ear is generally considered to be a more acceptable site than the mouth or rectum for temperature measurement, for use of the external auditory canal eliminates common problems such as breakage or perforation of the rectal wall or biting or gagging on a probe placed in the mouth. As early as the 1960's, thermistors in contact with the tympanic membrane were routinely used in the treatment of severely burned patients. However, because of the risk of injury and inconvenience of application, such contact type temperature sensors have not been widely used to measure temperatures in the ears of awake, alert patients.
Noncontact infrared thermometry differs from the above-referenced contact thermometry in that a sensor is placed at the external opening of the auditory canal for sensing the infrared energy emitted from the tympanic membrane, without contacting the tympanic membrane. Noncontact infrared thermometry is routinely used in industry to remotely measure process and machinery temperatures, and techniques for this purpose are described in detail in an article by C. Hamel entitled "Noncontact Temperature Sensing With Thin Film Thermopile Detectors," SENSORS, Jan. 1989, pages 28-32. However, although noncontact infrared thermometry is common in industrial applications, this technology has only recently been applied to medical temperature measurements.
In clinical application, the end of the probe portion of a noncontact infrared sensor must be small enough to be placed in the outer portion of the auditory canal (just past the cartilage of auricula), where the sensor can get a clear "view" of the tympanic membrane. Placing such a sensor requires only a little training and is very similar to the maneuver used to visualize the eardrum with an otoscope. Typically, there is no risk of eardrum injury because the probe is not long enough or small enough to be inserted past the mastoid process. Moreover, normal amounts of cerumen (ear wax) in the auditory canal typically do not interfere with an accurate temperature reading.
The most advantageous feature of infrared tympanic thermometry is that it takes very little time. Temperature readings typically can be taken in a few seconds. Speed is inherent in infrared thermometry because the sensors measure emitted infrared radiation instead of being brought into contact with the body until thermal equilibrium is reached as with typical oral thermometers. The speed means less discomfort to patients, which is especially important for children and in emergency situations. Another widely appreciated advantage over conventional thermometers is that tympanic thermometry uses the ear, which is less likely to harbor pathogens than the mucous membrane-lined mouth or rectum.
An early device for such a method of measuring body temperature using infrared emissions from the tympanic membrane is disclosed by Barnes in U.S. Pat. No. 3,282,106. Barnes therein discloses the general configuration of a tympanic membrane directed infrared thermometer which is inserted into the auditory canal so as to sufficiently enclose the detector apparatus such that multiple reflections of the radiation from the tympanic membrane transform the auditory canal into a "black body" cavity, a cavity with emissivity theoretically equal to one. However, in his early example of a noncontact infrared tympanic thermometer, Barnes does not consider, or in any way describe, how the device of that patent is calibrated, how accuracy is maintained for clinical use, or how contamination of the instrument by cross-infection can be prevented. Such problems have restricted the usefulness of the tympanic thermometer of Barnes in clinical settings.
Subsequent patents have addressed the problems with the thermometer of Barnes. For example, O'Hara et al. describe in U.S. Pat. Nos. 4,602,642 and 4,790,324 a noncontact infrared tympanic thermometer which addresses the calibration issues left unexplored by Barnes. In fact, the infrared sensor of O'Hara et al. is calibrated before each use. For this purpose, the sensor is housed in a temperature controlled cavity and special care is taken to maintain a constant temperature in the waveguide which directs the infrared emissions from the tympanic membrane onto the detector. In particular, the hand held probe unit of the tympanic thermometer of O'Hara et al. has an infrared sensitive thermopile mounted in a metal housing which is kept at a constant reference temperature by a regulator circuit. A waveguide tube surrounded by a thermally insulative probe directs infrared emissions from the tympanic membrane to the thermopile. The thermopile and regulator circuit of the probe unit are then electrically connected to a processing circuit in a chopper unit. Prior to taking a patient's temperature, the probe unit is mated with the chopper unit so that the thermopile detects infrared emissions from a reference target which is also kept at a constant reference temperature by another regulator circuit. The processing circuitry repeatedly acquires the output level of the thermopile and stores calibration data. The probe unit is then removed from the chopper unit, the probe is covered with an infrared transparent disposable speculum and is inserted into the patient's external ear canal. The patient's core temperature is then determined by comparing the stored calibration data to the maximum output of the thermopile during a succession of auditory canal samplings.
Another technique for calibrating a noncontact infrared thermometer is disclosed by Berman et al. in U.S. Pat. No. 4,784,149. Berman et al. therein disclose an automatic calibration technique for an infrared thermometer whereby the housing of the device is provided with a chamber shaped to receive the probe and a target for viewing by the infrared sensor when the probe is in the chamber. An error signal is thereby generated which is added to the output signals of an ambient temperature sensor and an infrared sensor within the probe when they view a body tissue for temperature measurement.
Fraden discloses in U.S. Pat. No. 4,797,840 a further calibration technique for an infrared thermometer in which a pyroelectric sensor in the thermometer housing is shielded from infrared radiation from exterior to the thermometer housing and is then selectively exposed to infrared radiation from the object to be measured to generate a first electrical signal related to the absolute temperature of the object to be measured. The ambient temperature of the pyroelectric sensor is then sensed and a second electrical signal proportional thereto is generated. The first and second electrical signals are then processed to calculate the temperature of the object to be measured. Errors due to temperature differences are minimized by thermally isolating the barrel of the sensor (which is in thermal equilibrium with the pyroelectric sensor) from ambient heat sources such as the human body by a protective thermoisolator coating. Calibration is accomplished by electrical calibration using a calibration circuit (FIG. 9) including a piezoelectric element which creates a mechanical stress calibration signal when a shutter is closed. The value of the electrical signal at the time of calibration of the thermometer is stored in memory. The shutter is then opened for temperature readings, and the resulting reading is adjusted by the stored value.
Wood discloses in U.S. Pat. No. 4,895,164 a technique for maintaining accuracy in clinical settings when using a noncontact infrared tympanic thermometer. During use, the radiation sensor is held in isothermic condition with a waveguide at ambient temperature. A thermistor or some other temperature sensor is thermally coupled to the radiation sensor for compensating for changes in ambient conditions. Also, the infrared radiation sensor of the device of Wood is constructed and configured so as to remain in an isothermic state, even during changes of ambient temperature, by positioning the infrared radiation sensor assembly within the housing so as to form an insulative air space between the housing wall and the isothermic assembly. Low emissivity barriers such as polished or gold plated aluminum tubing are also placed around the protruding portion of the waveguide in order to insulate the waveguide to limit the effects of temperature changes. Thus, accuracy is maintained in the device of Wood by maintaining a thermally stable environment around the detector.
On the other hand, Beckman et al. disclose in U.S. Pat. No. 4,900,162 a radiometer thermometry system for measuring the temperature of a target such as a tympanic membrane in which the temperature of the radiation detector can be changed so as to minimize the difference between the temperature of the target (tympanic membrane) and the temperature of the radiation detector (ambient temperature). The target temperature is thus detected by a sort of successive approximation (null seeking) technique. A related technique is taught by Egawa et al. in U.S. Pat. No. 4,932,789, who teach preheating the probe to a reference temperature close to normal body temperature despite the ambient temperature.
Junkert et al. disclose in U.S. Pat. No. 4,722,612 an attempt to stabilize the detector rather than the detector's environment. In the device of Junkert et al., an optical blocking baffle is placed over one-half of a two element pair so as to render that half insensitive to incoming radiation. The thermopile detectors are connected in series opposition and adjacent to each other. Both detector halves are sensitive to ambient temperature, and since only one detector is sensitive to incoming radiation, the Junkert et al. detector is "stabilized" against some ambient temperature fluctuations. However, by totally blocking the radiation falling on a portion of the detector in this manner, Junkert et al. cause a temperature gradient to be generated in the detector substrate, thereby inducing errors. Moreover, by blocking radiation at the substrate level, unwanted radiation from the walls of the detector package is not eliminated and thus causes further errors.
Devices for preventing contamination of a tympanic thermometer because of the accumulation of ear wax and the like are also known. For example, O'Hara et al. disclose in U.S. Pat. No. 4,662,360 a protective, disposable speculum for use with an infrared tympanic thermometer. The device includes an infrared transparent window and method for manufacturing it in a manner that would insure repeatable infrared transmission properties through the window, for if the transmission properties vary from unit to unit the calibration of the instrument would not be stable. Since the device is disposed of after each use, cross-contamination by ear wax and the like is avoided. Another example of a disposable speculum is disclosed by Twentier in U.S. Pat. No. 3,878,836, while a speculum for use with an otoscope is described by Kieffer, III et al. in U.S. Pat. No. 4,380,998.
Although prior art infrared thermometers of the type described above have addressed many of the shortcomings of the Barnes thermometer, several problems remain. For example, since the prior art noncontact infrared thermometers propose to maintain accuracy primarily by maintaining a thermally stable environment around the detector during operation, their accuracy is limited by the ability to maintain isothermic conditions in areas where there may be substantial temperature differences. In other words, since the environment around the detectors may not be made truly isothermic, more accurate detectors which need not be maintained under such conditions and which do not induce temperature gradients are desired for clinical use.
Reliable infrared detector devices which are extremely accurate in clinical use have been previously developed by the present inventors for use in a capnograph. As described in our U.S. Pat. application Ser. No. 07/401,952 (abandoned). 07/522,208 and 07/522,177 (now U.S. Pat Nos. 5,095,913 and 5,081,998, respectively), we have previously developed a detector device which eliminates thermal drift of thermopile detectors used in detecting the concentration(s) of at least one gaseous component of gases expired by a patient. The apparatus described in those applications comprises at least two series opposed infrared detectors which generate electrical signals when illuminated by optical energy provided by an infrared radiation source. In other words, the detectors are connected so that their outputs are subtracted. Outputs from the detectors are thus possible only if the same energy does not fall equally on both detectors. For this purpose, means for attenuating the infrared energy illuminating at least one of the detectors is provided. An optically stabilized signal is thereby produced since the undesired infrared signals fall upon both detectors equally and are effectively cancelled out, while the desired signal is maintained because of the difference provided by using the attenuating means over one of the optical detectors. The resulting information may then be processed and displayed as representative of the concentration of elements such as CO.sub.2 expired by a patient. However, no such technique has previously been used to overcome the above-mentioned problems with noncontact infrared thermometers so as to permit more widespread clinical use of such devices.
Accordingly, it is desired to overcome the aforementioned problems with prior art noncontact infrared tympanic thermometers by utilizing optically stabilized infrared detectors. The present invention has been designed to adapt our above-mentioned optical stabilization techniques into noncontact infrared tympanic thermometers so as to improve their accuracy without adversely affecting their ease of use.